The present invention relates to a polyurethane base lens resin, polyurethane base lens and processes for preparing the same. Plastic lenses have been rapidly popularized in recent years for optical elements such as lenses of eyeglasses and cameras. Plastic lenses are lightweight, are less fragile than inorganic lenses and are capable of being dyed with ease. 2. Description of the Prior Art
An example of a resin presently widely used for such lenses is a radical polymerization product of diethylene glycol bisallyl carbonate (hereinafter abbreviated as DAC). The resin has excellent impact resistance, is lightweight, has prominent dye-affinity and good machinability including cutting ability and polishing ability.
However, lenses prepared from the resin have a smaller refractive index (n.sub.D =1.50) than inorganic lenses (n.sub.D =1.52). In order to obtain equivalent optical properties to glass lenses, it is necessary to increase the center thickness, peripheral thickness and curvature of the lens and hence the lens as a whole becomes inevitably thick. Therefore, resins providing a higher refractive index are desired.
Other resins that have been known to provide plastic lenses having high refractive indexes are polyurethane base resins obtained by the reaction of isocyanate compounds with hydroxyl compounds such as diethylene glycol (disclosed in Japanese Patent Laid-Open No. 136601/1982, U.S. Pat. No. 4443588), with halogen containing hydroxyl compounds such as tetrabromobisphenol-A (Japanese Patent Laid-Open No. 164615/1983), with sulfur containing hydroxyl compounds (Japanese Patent Laid-Open Nos. 194401/1985 and 217229/1985, U.S. Pat. No. 4680369, U.S. Pat. No. 4780522), and with polythiol compounds (Japanese Patent Laid-Open Nos. 199016/1985 and 267316/1987, U.S. Pat. No. 4689387).
The lenses prepared from these resins, however, are still exhibit unsatisfactory refractive indexes, although the indexes of these lenses are better than that of a lens prepared from DAC. Further, since these resins are prepared from compounds containing many halogen atoms or aromatic rings in a molecule to improve the refractive index, lenses obtained from these resins have disadvantages such as large dispersion of refractive index, poor weatherability and high specific gravity.
In molding the polyurethane base lens, it is usually difficult to release the polymerized lens from the mold due to good adhesion between the polyurethane compounds and the mold. In order to improve the mold releasing property, the present inventors have previously proposed a method for using external mold releasing agents (Japanese Patent Laid-Open No. 267316/1987) and a method for applying polyolefin resin molds (Japanese Patent Laid-Open No. 236818/1987).
The above methods, however, are still insufficient for improving the mold releasing property in the casting polymerization of the polyurethane base lens.
When external mold releasing agents are used, a portion of the agents used for treating the inner surface of the mold migrates to the surface and interior of the polymerized lens which results in problems such as nonuniformity of the surface of the lens and turbidity. When a mold is used repeatedly, mold releasing treatment is required in each molding cycle. Such treatment lowers productivity in producing the lens and thus is extremely uneconomical in industrial production.
When the polyolefin resin molds are applied, the resin molds are deformed at elevated temperatures which causes large profile irregularities on the molded lens. Therefore it was found that the resin molds could not be applied to articles requiring high accuracy on the molded surface.
An example of the use of internal mold releasing agents to improve releasing property of molded materials, is a method for incorporating butyl phosphate in DAC. However the internal mold releasing agents cause adverse effect on the appearance of molded articles [S. Mima, Polymer Digest, 3, 39 (1984)], and hence have not been positively used.